Connectivity Fault Management (CFM), as described in IEEE 802.1ag, is a key component of operation, administration, and maintenance for carrier Ethernet. IEEE 802.1ag specifies protocols, procedures, and managed objects for end-to-end fault detection, verification, and isolation. IEEE 802.1ag establishes managed objects, called Maintenance Associations (MAs), to verify the integrity of a single service instance by exchanging CFM messages. The scope of an MA is determined by its Management Domain (MD), which describes a network region where connectivity and performance is managed. Each MA associates two or more Maintenance Association Endpoints (MEPs) and enables Maintenance Association Intermediate Points (MIPs) to support fault detection and isolation.
A continuity check protocol is used for fault detection. Each MEP periodically transmits Connectivity Check Messages (CCMs) and tracks CCMs received from other MEPs in the same maintenance association.
When a MEP managed object belonging to an MA is created, its Continuity Check Receiver maintains one instance of the Remote MEP state machine for each of the other MEPs configured for this MA. The MEP Continuity Check Receiver utilizes Remote MEP variables and state machines to track each Remote MEP. This state machine monitors the reception of valid CCMs from a Remote MEP with a specific MEPID. It uses a timer that expires in 3.25 to 3.5 times the length of time of CCM interval. If no CCM is received within the timer expiration period from a remote MEP, the Remote MEP state machine of this Remote MEP detects a defect, which can in turn generate a Fault Alarm.
A problem arises when the process specified in the current IEEE 802.1ag-2007 standard is applied to an Ethernet Tree (E-Tree) type of service instance, such as a Provider Backbone Bridge-Traffic Engineering (PBB-TE) Point-to-Multipoint service instance. In an E-Tree type of service instance, there is a root node which communicates with a plurality of leaf nodes. There is a MEP located in the root node and in each leaf node. All of these MEPs belong to the same MA, and according to IEEE 802.1ag, each MEP must be aware of all the other MEPs in the same MA, and the Continuity Check Receiver in each MEP must maintain one instance of the Remote MEP state machine for each of the other MEPs. Additionally, the Remote MEP state machine instance in each Continuity Check Receiver is required to monitor the reception of valid CCMs from each remote MEP, and indicate a defect if no CCM is received for a predefined period of time from a given MEP. As shown in FIG. 1, however, in an E-Tree type of service instance, MEPs in the leaf nodes cannot receive each other's CCMs because they only communicate with the root MEP. Therefore, if they comply with IEEE 802.1ag, the Remote MEP state machine instance in each leaf will indicate defects in the other leaf MEPs due to failure to receive their CCMs even when their operation is normal.